In the operation of an internal combustion engine, the engine is in the state where misfire can readily take place when the engine begins to decelerate while the throttle valve of the engine is kept in its fully closed position. This is because a very rich air-fuel mixture, which cannot be ignited in the combustion chamber, is introduced into combustion chambers of the engine and, therefore, an unburnt air-fuel mixture is discharged into the exhaust system. The thus discharged unburnt air-fuel mixture, together with secondary air introduced into the exhaust system by an air injection system of the engine, is burnt in a catalytic converter arranged in the exhaust system to cause an explosive after-fire. As a result, the pressure in the exhaust system is elevated and exhaust gas is discharged from an exit of the exhaust system at sonic velocity. Therefore, not only a very large explosion is generated, which pollutes the sound environment around the vehicle, but also, damage is caused to parts of the exhaust system, for example the catalytic converter and muffler.
As means for preventing the undesired after-fire, there is already known an apparatus which has a vacuum operated air control valve which is opened for a predetermined time to introduce an amount of air into the intake system of the engine, when the engine begins to decelerate while the throttle valve is kept in its fully closed position. As a result of the introduction of this air, the very rich air-fuel mixture is diluted for proper combustion in the combustion chamber. Thus, the discharging of the unburnt air-fuel mixture into the exhaust system is prevented and, therefore, generation of the after-fire can be avoided.
However, in the above-mentioned known apparatus, the air control valve is opened for introducing the air into the intake system when the engine begins to decelerate, even if the engine is in a warming up operation. While warming up, the engine is in a very unstable state for complete combustion of the air-fuel mixture in the combustion chamber, due to the fact that the temperature of the engine is not yet fully elevated. Consequently, as a result of the unstable state of the engine during a warming up operation, the engine is easily stopped or abnormally operated if the air control valve is opened to introduce air into the intake system during such operation.